The latter part of the twentieth century has seen a substantial growth in the number of ordinary citizens participating in regular exercise. The reason for this is that lack of exercise has been discovered to be associated with ill health and premature death due, for example, to heart disease. Being overweight is an additional contributory factor. Whereas in the middle of the twentieth century being overweight was accepted as not abnormal in people of middle age, it is now unfashionable to be fat, to take unhealthy habits such as smoking and drinking to excess, and to be unfit or "out of shape".
Greater study of human fitness has shown that there are separate identifiable forms of exercise which have their own purpose and effect. One of the simplest forms of exercise is running which, without imposing too much strain on the body, enables the performer to expend his or her energy and raise their heart rate during the exercise period so as to improve their cardiovascular condition.
There are other forms of exercise based on running such as in many competitive sports. The level of expended effort in playing is affected by the game played and is therefore not wholly within the participant's control. Swimming is another form of cardiovascular exercise which, however, although easier than running in terms of strain on body joints is not as useful to the populace as a whole because it requires a swimming pool or the like which may not be conveniently available, and there can be breathing problems when the participant is short of breath and partially under water.
Another form of exercise which is not primarily intended to increase cardiovascular fitness to any great extent but does improve muscle tone and strength is resistance exercise where a participant flexes his or her muscles against the resistance of weights, springs or, in isometric exercise, statically against the resistance of other muscle groups in the body or an immovable object.
Ideally a combination of these forms of exercise, running and resistance exercise, gives the best overall result, but sometimes in practice such a combination is difficult to achieve. For example, in icy weather conditions, running is dangerous so an exerciser may be restricted to an indoor weight training apparatus. Equally an exerciser may be away from home and not readily able to find a weight training apparatus although they are able to run. Similarly time constraints may prevent a user from using an indoor exercise apparatus and in addition also going outdoors for a run.